It doesn't go for everything. For example, some people like something, like a piece of candy, and some people don't. But there are some sensations that evolution has developed in order for us to keep doing them, and i said that this might be a reason why streching feels good

"As a biologist, I firmly believe that when you're dead, you're dead. Except for what you live behind in history. That's the only afterlife" - J. Craig Venter

I agree with the idea that your brain has developed a mechanism to interpret stretching as a pleasurable sensation because it is beneficial..... I bet in the days of old, when this trait was not widespread among our older ancestors some caveman did not interpret stretching as beneficial and went on to party without stretching and they pulled a muscle or something..... However it is important to consider that the morning stretch is the most pleasurable, based on this you might think that the bigger the stretch the more pleasurable the sensation.... This is probably true to a certain extent, since we do stretch our tendons and muscles during our normal activities of the day, like when we jumped and reached for prey during our species early days, in this case the brain is not meant to recognize that particular type of stretch as pleasurable, because it is awfully distracting, so when the brain is focused in another task, the stretch is not perceived as pleasurable..... However on the type of early morning stretch, when you have no worries and your brain is not focused on other activities, the larger the stretch the more stretch sensitive cation channels will open in propioceptive neurons of the golgi organ and muscle spindles, this influx of cations will allow the propioceptive neurons to reach action potential firing threshold and fire away, this signals will synapse at the spinal cord, where it later must reach higher cortical areas and synapse with neurons that carry one of the many feel-good neurotransmitters that the good lord in his immense knowledge of neurochemistry, bestowed upon us One more thing, I doubt that it has to deal with the circulation, since other activities that increase blood flow, like serving some fools on the the dance floor at the tune of la Macarena, does not lead to that good feeling you get in the morning after a good stretch....Peace out